As a technique to make an observer to perceive a stereoscopic image, a stereoscopic image displaying method based on a binocular disparity, wherein the position difference of the left and right eyes of the observer is used to good advantage, is generally used. This method is an application of the principle used for a stereogram which makes the left and right eyes of an observer separately perceive different two-dimensional images so as to make the observer's brain recognize a three-dimensional image based on the difference of the perceived images. As such the stereoscopic image displaying method, there are known a stereoscopic viewing method using glasses and a stereoscopic viewing method with naked eyes, using no glasses. The stereoscopic viewing method with naked eyes includes a multi-viewpoint type and a two-viewpoint type, depending on the number of viewpoints of an observer.
To represent a stereoscopic image through the naked-eye method by using a two-dimensional image displaying device such as a normal flat-panel display device, there is used a method to spatially separate images for left and right eyes displayed on a two-dimensional screen and present the images to the left and right eyes separately, by using one of a parallax-barrier method wherein a light-shielding structure (barrier) formed by slits is arranged between a display panel and a light source, and a lens method wherein lenticular lenses are arranged between the observer and a display panel for displaying two-dimensional images.
On the other hand, in many of those stereoscopic viewing methods with naked eyes, the barrier or the lenses are fixedly joined to a display panel, which restricts a stereoscopic perceptive region into a narrower area. Further, in a situation where the number of viewpoints is increased so as to widen the stereoscopic perceptive region in the multi-viewpoint method, it is required to display an increased number of images on the display panel according to the number of viewpoints, which causes deterioration in resolution of each image. When an observer moves under the condition that the stereoscopic perceptive region is restricted to a narrow area, the viewpoint of the observer easily goes out from the stereoscopic perceptive region and easily enters in a pseudoscopic region in which the observer perceives the images for left and right eyes inversely. In this situation, the observer hardly perceives stereoscopic images.
In view of the above matter, Japanese Unexamined Patent Application Publications (JP-A) Nos. 1107-38926 and 2005-175973 disclose a head-detecting device for detecting a spatial position of the head of an observer, and a servo mechanism configured to mechanically move a lenticular lens composed of arrayed cylindrical lenses or a light-shielding structure serving a barrier to be synchronized with the output of the head-detecting device, so as to shift the stereoscopic perceptive region with following the viewpoint of the observer. Further, Japanese Patent (JP-B) No. 4495982 (JP-A No. 2005-223727) discloses a technology to display plural images through a time divisional method, to drive a polarization switching element formed of liquid crystal to be synchronized with the displayed images, and further to use both of a barrier and lenses, for a purpose of achieving an increase of viewpoints of a display device with a smaller number of pixels.
Further, in a field of stereoscopic image display devices, there is a strong demand to achieve a two-dimensional (2D) display for mainly representing letters and characters and a three-dimensional (3D) display for mainly representing objects and landscapes on one screen in a mingling manner. As a method to switch the display mode of one pixel between 2D and 3D, there have been proposed a lenticular-lens method using an array of sub-pixels elaborated with the demand considered in JP-B No. 4400172 (JP-A No. 2004-280052) and a liquid-crystal lens method by which the lens properties can be turned on/off in JP-A No. 2009-104137 and JP-B Nos. 4687073 and 3940725 (JP-A Nos. 2006-0126721 and 2004-258631).
Regarding the lenticular-lens method, assuming that the extending direction of the ridgelines of the lenticular lens is the vertical direction, the vertical pitch and the horizontal pitch of each pixel are the same and sub-pixels for R, G and B colors are arranged in vertical stripes in parallel with the extending direction of the lenticular lens, in conventional arts. Therefore, in the conventional arts, the horizontal resolution has been required to be doubled in order to secure the 3D resolution to be equivalent to the 2D resolution. JP-B No. 4400172 discloses a structure that the horizontal pitch of each pixel is set to half of the vertical pitch and sub-pixels for R, G and B colors are arranged in horizontal stripes perpendicular to the extending direction of the lenticular lens, which achieves a display wherein 2D display and 3D display mingle together, with securing the 2D and 3D resolutions at the same level.
Regarding the liquid-crystal lens method, a liquid-crystal lens is a kind of optical element which takes advantage of physical properties of liquid crystal materials, such as an electro-optical effect and a great anisotropy in refractive index, and which is employed to make the best use of its advantages that a low drive voltage, low power consumption, and two-dimensional arrangement of the elements can be easily achieved (please see the following non-patent literatures: S. Sato, Jpn. J. Appl. Phys., Vol. 18, No. 9 pp. 1679-1684, 1979; Matsuda et al., Appl. Opt., Vol. 36, No. 20, pp. 4772-4778, 1997; Nose et al., Jpn. J. Appl. Phys., Vol. 39, No. 11, pp. 6383-6387, 2000; and Lu et al., J. Display Technol., Vol. 7, No. 4, pp. 215-219, 2011). For example, a variable-focus liquid-crystal lens is achieved by preparing a liquid-crystal cell which employs an electrode substrate with a surface in a lens shape, and applying a voltage to the liquid-crystal cell so as to change the refractive index of the liquid crystal in the polarization direction of incident light from “ne” to “no”.
Further, a liquid-crystal micro-lens is composed of a liquid-crystal cell which includes two flat substrates on which a slit-patterned electrode or a circular-hole-patterned electrode is arranged and includes liquid crystal sealed between the substrates. When a voltage is applied to the electrode, a non-uniform electric field in axial symmetry is generated in a slit-patterned or circular-hole-patterned opening where the electrode does not present. Molecules of liquid crystal are oriented along the generated electric field again, and the refractive-index profile of the liquid crystal has a shape of the binominal profile in which the refractive index continuously changes from “ne” to “no” in the area from the center of the opening to the patterned-edge section. This refractive-index profile causes optical-path difference Δnd provides a lens effect. In other words, a variable-focus GRIN lens is provided.
Such a liquid-crystal lens can be formed to have a refractive-index profile which has a shape of the almost ideal binomial profile, which provides excellent lens properties. By using divided electrodes and/or an externally-controlled electrode structure, the refractive-index profile can be shifted parallel along the plane of the liquid-crystal cell, in other words, the lens position can be shifted, by about 10 μm (hereinafter, μm is also represented as “um”), with securing the lens properties (see, the above-described non-patent literatures: Matsuda et al. and Nose et al.). However, the opening which presents lens properties has been fixed by the initial electrode arrangement, and a shift of the lens can cause a lack of a part of the lens. Therefore, this structure hardly provides the shift amount of the lens as large as a few hundred micrometers by which the lens can perform a viewpoint tracking.
Further, the liquid-crystal lens requires a sufficient optical-path length, which makes the cell gap “d” larger than the cell gap of a general liquid-crystal display panel, in order to increase the retardation (Δnd) of the liquid-crystal lens. Thereby, the response of the liquid-crystal lens becomes relatively slow. This matter can be enhanced if liquid crystal with sufficiently-great anisotropic refractive index Δn is available. However, the anisotropic refractive index Δn of currently available liquid crystal is about 0.2 at most, which makes the enhancement difficult. In view of that, the liquid-crystal lens disclosed in Lu et al. has a refractive-index profile in a form of a Fresnel lens, to achieve a reduction of the cell gap.
JP-A No. 2009-104137 discloses the following stereoscopic image display device. Under the situation that a liquid-crystal lens is used in place of a lenticular lens in a stereoscopic image display device, the liquid-crystal lens is required to have a refractive-index profile equivalent to that of arrayed cylindrical lenses. In the stereoscopic image display device, the widths and intervals of the electrodes are changed in order to obtain excellent liquid-crystal lens properties also in a space between neighboring lenses, in other words, around the edge parts of the liquid-crystal lens under the above situation.
JP-B No. 4687073 (JP-A No. 2006-126721) discloses the following liquid-crystal lens in order to adjust the refractive-index profile for each pixel independently. In the liquid-crystal lens, gaps and widths of strip-shaped electrodes are changed gradually in one direction so as to make an electric field gradient, thereby, the liquid-crystal lens has a prism-shaped refractive-index profile. In each pixel, a constant drive voltage is applied to stripe-shaped electrodes and the counter electrode serves as a common electrode.
JP-B No. 3940725 (JP-A No. 2004-258631) discloses a technique to use a liquid-crystal lens in place of a lenticular lens, and a phase modulation unit (composed of a half-wavelength plate and a ferroelectric-liquid-crystal cell) configured to rotate a polarization plane for switching the display mode between 2D and 3D, so that the 2D/3D display modes can be switched quickly by using the phase modulation unit rather than the on/off switching of the liquid-crystal lens.
JP-B No. 3814366 (JP-A No. H10-221703) discloses a way to provide liquid-crystal lenses formed by the following method in order to control the liquid-crystal lenses individually for each pixel. There is provided a high-resistant thin-film resistance wire with joined to low-resistant stripe-shaped electrodes (transparent electrodes made of metal or low-resistant ITO (Indium Tin Oxide)) so as to tie up the low-resistant stripe-shaped electrodes. Different voltages are applied to the both ends of the high-resistant thin-film resistance wire to form a gradient electric-field distribution and provide a liquid-crystal lens. By joining the stripe-shaped electrodes together with the thin-film resistance wire and applying different voltages to the respective stripe-shaped electrodes, the number of output voltage values is restricted and cost of the driver is reduced.
JP-A No. 2010-56712 proposes the following method to maintain a stereoscopic view. It is assumed that the optical axis going through a panel plane of a stereoscopic image display panel (a plane including the z-y axes) perpendicularly is defined as the x-axis. There is provided an image separating element (such as a lenticular lens, a parallax barrier and a liquid-crystal lens) for spatially separating an image for the right eye and an image for the left eye. Even when a posture of a user who is observing the panel changes so as to be rotated in the O-direction within the panel plane, the image separating element is rotated according to a detection result, to maintain a stereoscopic view.
JP-A No. 2002-328333 discloses the following display device using an optical wavefront control. In the display device, plural microscopic liquid-crystal cells are arrayed in matrix and are separately controlled to obtain a binocular disparity. The JP-A No. 2002-328333 further discloses, as another structure to control liquid crystal by using just an electrode structure in place of the structure to control liquid crystal as the individual cells, a simple matrix structure that electrodes formed on opposite substrates are arranged to cross each other (see paragraph 0061 of the document).
However, in the above-described conventional arts, the structures of JP-A Nos. H07-38926 and 2005-175973 require a servo mechanism configured to move a lens and a space required for moving the lens, for a purpose of the viewpoint tracking, which causes a problem that the device becomes greater in size.
The technique disclosed in the JP-B No. 4495982 is a technique just for achieving multiple viewpoints, which lacks a function to tracking a viewpoint and is required to be coupled with a lenticular lens. Further, as for a liquid-crystal cell having a wedge-shaped surface, its surface processing is difficult, as can be seen from the description of the document: “it is difficult to sharpen the apex of the saw tooth formed on the saw-tooth substrate, and in general, the apex has a certain radius of curvature. For this reason, the step portion of the saw tooth may scatter light, which may degrade the pixel image transmitted through the step portion.” (see paragraph 0033 of the document). Further, a rubbing processing of the substrate surface for orienting liquid crystal molecules can be performed insufficiently at the step portion, especially at the bottom of the wedge shape, which can cause abnormal arrangement of liquid crystal molecules. Therefore, light can be scattered also at the bottom of the wedge. Furthermore, the lenticular lens and the liquid-crystal polarization cell are required to be made of the same material, otherwise distortion of those elements is caused due to the difference of their shrinkage factors, in a case that the structure is increased in size, and such the situation can make a viewpoint displacement and an abnormal display coming from leakage of scattered light. To solve the problem, a method to use a silica glass for the substrate can be given. However, general silica glass is so expensive to be hardly used in large-sized products, which is also a problem.
In the structures disclosed in Matsuda et al. (Matsuda et al., Appl. Opt., Vol. 36, No. 20, pp. 4772-4778, 1997) and Nose et al. (Nose et al., Jpn. J. Appl. Phys., Vol. 39, No. 11, pp. 6383-6387, 2000), the lens-shift amount is so small as to be insufficient to track a viewpoint, which is a problem. Further, Matsuda et al. and Nose et al. do not propose any idea to apply their techniques to a stereoscopic image display device.
Lu et al. (Lu et al., J. Display Technol., Vol. 7, No. 4, pp. 215-219, 2011) discloses the structure using stripe-shaped electrodes to establish a refractive-index profile in a Fresnel-lens shape in order to make the cell gap thin. The form of a Fresnel lens is so sharp to be hardly realized by a liquid-crystal lens with accuracy, and such a liquid-crystal lens can bring a deterioration of image-forming properties and an increase of stereoscopic crosstalk. Further, a liquid crystal layer is put within the cell so as to induce a homogeneous alignment of liquid crystal molecules so as to be parallel with the direction of stripes of the transparent electrodes. Therefore, when a potential difference of neighboring stripe-shaped electrodes becomes large, molecules of liquid crystal rotate in the direction perpendicular to the direction of stripes which can disturb the alignment of liquid crystal molecules. Further, in the structure, a voltage applying pattern has been established so as to realize the optimum lens properties for the initial lens alignment and the initial lens pitch without estimating a lens shift. The structure of the document just switches 2D display and 3D display, and neither a system to tracking a viewpoint nor a similar idea is proposed in the document.
The structure disclosed in JP-A No. 2009-104137 has stripe-shaped electrodes whose intervals and widths are not uniform. Therefore, it is difficult to shift the refractive-index profile within the substrate plane (namely, in an x-y plane or the horizontal direction) with keeping its form. Further, it is difficult to track a viewpoint in this structure because there is no idea to shift a lens in the document.
The structure disclosed in JP-B No. 4687073 has stripe-shaped electrodes whose intervals and widths are not uniform. Therefore, it is difficult to shift the refractive-index profile within the substrate plane (namely, in the horizontal direction) with keeping its form. Further, it is difficult to control the refractive-index profile at a midway point of neighboring pixels, because the lens pattern is independent for each pixel. Therefore, it is difficult to track a viewpoint in this structure because there is no idea to shift a lens in the document.
The structure disclosed in JP-B No. 3940725 has a liquid-crystal lens wherein an electrode is arranged only at the edge section thereof. Therefore, the electrode is fixed and is hardly moved within the substrate plane (namely, in the x-y plane or the horizontal direction). Further, it is difficult to track a viewpoint in this structure because there is no idea to shift a lens in the document.
The structure disclosed in JP-B No. 3814366 has a liquid-crystal lens wherein stripe-shaped electrodes are connected with a resistance wire, the voltage is hardly adjusted on each electrode separately, and the lens pitch is determined by the length of the resistance wire. Therefore, it is difficult to shift the refractive-index profile in the horizontal direction and is difficult to track a viewpoint in this structure because there is no idea to shift a lens in the document.
The structure disclosed in JP-A No. 2010-56712 uses a liquid-crystal lens as an image separating element. Regarding the structure of the liquid-crystal lens, pixel electrodes are arranged in matrix, and a liquid-crystal lens panel on which dots are formed is composed of the pixel electrodes, electrodes on a counter substrate and liquid crystal. The pixel electrodes electrically control the refractive index of the liquid crystal at each of the dots independently so as to rotate the refractive-index profile. However, each dot is required to be driven with an active element such as a TFT (Thin-Film Transistor), which increases the number of TFT elements, wires and outputs of the driver forming the liquid-crystal lens panel corresponding to an increase of the resolution of the image display panel and further brings a reduction of the yield, a reduction of the aperture ratio, and an increase of cost. Further, there is not proposed a method to translate the refractive-index profile under the condition that an observer moves in the horizontal direction of the display panel plane (the y-z plane), for example, in the y-axis direction, to go away from the rotation axis (the x-axis). Therefore, it is difficult to keep the stereoscopic view in the disclosed structure under such the condition.
In the structure of JP-A No. 2002-328333, separating microscopic liquid-crystal cells with light-shielding members makes the manufacturing processes complicate and causes a reduction of the yield. Further, controlling each of the cells with a TFT element separately can cause an increase of cost. In the microscopic cells, since liquid crystal in a boundary area does not move, a smooth form of the refractive-index profile is hardly obtained when a lens function is obtained by combining plural cells, which is insufficient as a light-wavefront control. In other words, such the structure may sufficiently converge light onto the position of an observer but forms an image which has a low quality, is distorted and has great aberrations. Further, there are no descriptions about the alignment conduction of liquid crystal in this document, and it is actually difficult to form a refractive-index profile which is so smooth to realize a stereoscopic image, just by applying voltages to the electrodes. If in a simple matrix structure under a uniform molecular alignment states which is as simple as that used in a general liquid-crystal lens, a changed voltage is applied to each of neighboring working areas (each being equivalent to a microscopic cell), a display pixel is affected by leakage electric fields coming from neighboring display pixels on the all sides including up, down, left, right and diagonals, as an external disturbance. This situation causes a disturbance of the molecular alignment of liquid crystal (disclination line) and brings a difficulty of an excellent control of the refractive-index profile.